The tRNAs from livers of old (24 month old) rats can accept less, per unit tRNA, of lysine, leucine, phenylalanine, serine, and valine than tRNAs from young (3 month old) rats. We plan to examine the extent to which the rest of the protein amino acids can be accepted. This low efficiency is associated with minor but reproducible changes in isoaccepting tRNAs, as judged by RPC-5 column elution. This phenomenon will be investigated in more detail. The old and young tRNAs act the same on 5% polyacrylamide gel electrophoresis, but act differently on 10% SDS-polyacrylamide gels. We will examine this difference in more detail via 2 dimensional gels. Glucose-6-phosphate dehydrogenase, which we have shown to have lower specific activity, a different isozyme pattern, and a slightly different thermal sensitivity, will be purified. The enzyme will be examined via isoelectric focusing and peptide mapping to see what types of changes occurred during aging.